Move DIP(WIndow) to Viewport conversion code for drag&drop from browser to renderer.

This is improvement to the original CL (https://codereview.chromium.org/1605143003). I should have done this way because most of conversion happens in RenderWidget/RenderViewImpl.

BUG=485650

Review URL: https://codereview.chromium.org/1703433002

Cr-Commit-Position: refs/heads/master@{#376280}
4 files changed